An optical fiber comprises a core and a cladding which surrounds the core and has a lower refractive index than that of the core.
Several methods for producing the optical fiber have been developed and are commercially employed. Examples of such methods are the modified chemical vapor deposition method (M-CVD method), the vapor axial deposition method (VAD method) and the outside vapor deposition method (OVD method). In these methods, the core is mainly made of SiO.sub.2 /GeO.sub.2 type glass, and the cladding is mainly made of SiO.sub.2.
Since light is intensively propagated through the core of the optical fiber, from an idealistic viewpoint, the core is to be made of SiO.sub.2 which has the smallest attenuation of light transmission. On the other hand, the cladding made of SiO.sub.2 containing an additive such as B.sub.2 O.sub.3 and fluorine for reducing the refractive index since it should have a lower refractive index than that of the core. Although B.sub.2 O.sub.3 may be added to the core of the optical fiber employed in some uses, it is not preferred to add B.sub.2 O.sub.3 to the core of the optical fiber through which light having a wavelength longer than 1 .mu.m is propagated since the presence of B-O bonds absorb light in far infrared range. In this respect, fluorine is an ideal additive for the cladding since it does not induce any absorption peak in a wavelength range used for the optical fiber. Thus, it is preferable to adjust the refractive index difference between the core and the cladding of the optical fiber by making use of SiO.sub.2 as the core material and SiO.sub.2 /F type glass as the cladding material. Such an optical fiber is disclosed in Japanese Patent Publication No. 15682/1980.
The optical fiber comprising the core made of SiO.sub.2 and the cladding made of SiO.sub.2 /F type glass is fabricated from a glass preform produced, for example, by accumulating fluorine added quartz glass in a glassy state around a quartz rod in its radial direction by means of thermal plasma, or by depositing fine particles of fluorine added quartz glass around a quartz rod in its radial direction by flame hydrolysis of the glass raw material and sintering the deposited fine particle mass to make it transparent, or by depositing fluorine added quartz glass on an inner surface of a quartz tube by the M-CVD method to form a core portion and collapsing the central hollow part to form a glass preform in a solid rod form.
However, by the thermal plasma method, it is difficult to fabricate an optical fiber having satisfactorily low attenuation of light transmission from the produced glass preform, since an interface between the quartz rod and the accumulated fluorine added quartz glass is polluted and the fluorine added glass itself is contaminated with hydroxyl groups. By the method in which the fine glass particles are deposited around the quartz rod by flame hydrolysis, it is also difficult to fabricate an optical fiber having satisfactorily low attenuation of light transmission from the produced glass preform, since the quartz rod is contaminated with the hydroxyl groups made from water adsorbed on the surface of the quartz rod or produced by the flame hydrolysis of the glass raw material. In this method, water or moisture is easily removed from the fine glass particles but hardly removed from the inside of the quartz rod. Although the M-CVD method can produce the glass preform from which an optical fiber having comparatively good properties, it is poor in productivity.